Solar power is a renewable resource that continues to generate interest as an alternative to fossil fuels. Solar photovoltaic panels have been historically inefficient in converting sunlight into electrical energy as compared to the cost of the panels themselves. As an alternative to photovoltaics, interest in collecting heat from the sun for use in steam driven power plants has grown. This process involves collecting heat from the sun using thermal or reflective panels. This heat is used to warm a heat transfer fluid which, in turn, transfers the absorbed heat to water to produce high-pressure superheated steam which can be used to power turbines in power plants. The cooled heat transfer fluid exits the steam generator and re-circulates to the collecting panels. To optimize efficiency, it is important to operate heat absorption by the heat transfer fluid at a suitably high operating temperature. The keys to improving the effectiveness of the heat transfer fluid are to provide a fluid that does not have an excessive vapor pressure at very high temperature, remains liquid at ambient temperature, and does not chemically degrade at operating temperatures.
The dominant concentrating solar power plant configurations use a specialized heat transfer fluid to carry thermal energy from the collector to a separate steam generator to ensure stable, safe operation. Parabolic trough systems, for example, use a linear parabolic concentrator with receivers in an evacuated enclosure to reduce convection losses. The heat transfer fluid is circulated through the receivers throughout the solar field. An ideal heat transfer fluid is pumpable at ambient temperatures and yet remains stable with a low vapor pressure at the highest operating temperatures. Current solar collector systems operate at temperatures up to 400° C., limited by the upper operational temperature limit of heat transfer fluids currently used in these systems. This limits attainable steam temperature to temperatures substantially lower than those obtained in coal-fired power plants (e.g., up to 650° C.). Therefore, the thermal efficiency of steam turbine generators in solar power plants is currently lower than the thermal efficiency achievable in modern fossil fuel power plants. A new advanced heat transfer fluid that remains liquid up to a maximum service temperature near 500° C. or above, with desirable fluid properties and satisfactorily low vapor pressure, and that additionally remains liquid at temperatures as low as 80° C. or even outdoor nighttime ambient temperature, would improve performance of parabolic trough concentrating solar power plants. Increasing the maximum operating temperature above the current limit (near 400° C.) increases the overall efficiency of the Rankine cycle electrical power generation from near 37% to about 41%, and allows an increase of 10% in electric power production from the same collection mirrors and the same solar energy input. Such an advance will be a key contribution in current work to improve the economics and efficiency of electricity derived from concentrating solar power plants. Therefore, it is desirable to develop concentrating solar power systems and methods having improved heat transfer fluids and operating strategies.
The liquids currently used in parabolic troughs (solar heat collecting devices) are organic fluids which are capable of being heated to 380° C.-400° C. under pressure. Researchers have attempted to use higher molecular weight fluids which will not evaporate at higher temperatures (500° C. and over), but these fluids tend to solidify above ambient environmental temperatures (10-30° C.), making it difficult to use them as heat transfer fluids as the liquid solidifies in the pipes of the heat exchange system when the system cools at night or in absence of solar input. Additionally, conventional heat transfer fluids may be organic, aqueous, molten salt, or molten metal. Many oils and other organic compounds have been useful. When exposed to high temperatures, however, many organics begin to decompose, either by degrading into smaller fragments or by producing higher-molecular weight deposits. The small fragments usually have higher vapor pressures and increase system pressure unduly, while higher-molecular weight deposits lead to fouling and decreased heat transfer efficiency. For high-temperature use, aromatic compounds are more stable against decomposition than aliphatic compounds. A mixture currently used for high thermal stability and wide operating temperature range is a mixture of biphenyl and diphenyl oxide. These compounds have almost identical vapor pressures, boiling at 258° C. at 1 atm. The respective melting points are 80° C. and 35° C., but a eutectic mixture of 26.5% biphenyl/73.5% diphenyl oxide has a melting point of 12° C., so the operating range may extend below ambient temperature. When the upper pressure limit is about 10 atmospheres, the eutectic mixture has an upper operating temperature limit of about 400° C. This mixture is commercially available as Therminol VP-1 (Solutia) and Dowtherm A (Dow). Chemical stability also becomes an issue at yet higher temperatures, as the biphenyl/diphenyl oxide system shows slow decomposition that produces small fragments including hydrogen. This combination of high vapor pressure and hydrogen generation thus prevents this heat transfer system from use in many metal heat transfer systems above 400° C. This biphenyl/diphenyl oxide system is used as the organic fluid, for example, in the Kramer Junction parabolic trough systems.
Beyond organic fluids, the alternatives are presently less attractive for solar power applications. Direct steam production can be accomplished in a parabolic trough collector but only at pressures above the critical point of water (220 bar). Molten salts can be used at temperatures of 500° C. or higher with low vapor pressure and other attractive heat transfer characteristics. Conventional salts freeze at high temperatures, however, often 200° C. or higher depending on the composition of the salt. The eutectic mixture of sodium nitrate, sodium nitrite, and potassium nitrate (marketed as Hitec salt) has one of the lowest melting points (142° C.). The freezing of a salt at an overnight low temperature is particularly inconvenient in a parabolic trough system, likely requiring a large amount of piping that would need to be heat-traced. Molten metals have desirable features for heat transfer at high temperature, but their high reactivity and handling hazards make them more suitable for sustained high-temperature use, such as for nuclear power, than for the widely varying temperatures experienced during the daily temperature cycle of a concentrating solar power system. Because of the high heat flux at the receiver surface, vapor-phase and boiling heat transfer fluids are not favored for heat collection at the focus of a concentrating solar power collector.
In the power tower configuration, energy collection occurs by directing a solar photon flux to a central tower receiver prior to heat collection by a heat transfer fluid for steam generation. The operating temperature can thus be higher if the fluid has sufficiently low vapor pressure and high thermal stability. The overnight heat losses are less severe, thus the minimum temperature is not as low as in parabolic trough systems. Therefore, both the parabolic trough and power tower configurations will benefit from improvements in the maximum service temperature of a new heat transfer fluid.
U.S. Pat. No. 2,910,244, issued on Oct. 27, 1959, discloses methods and apparatus for the transfer of heat by means of a molten heat transfer medium which is solid at room temperature. This reference discloses incorporating a volatile solvent to the medium under pressure and in an amount sufficient to maintain the liquid phase in the system on cooling down. Specific mixtures include a water soluble salt or salt mixture and water.
U.S. Pat. No. 4,278,073, issued on Jul. 14, 1981, discloses a system, method and apparatus for converting solar energy to heat and shaft work utilizing a mixture of two compatible fluids. One fluid has a low boiling point, and the other has a high boiling point. The high-boiling point fluid disclosed in this patent has a boiling point no less than 250° F. High-boiling fluids exemplified in this reference include Dowtherm J, Therminol, and glycols and exemplified low-boiling fluids include Freon and alcohols.
It will be appreciated from the foregoing that there is currently a need for improved methods and systems providing heat transfer fluids that remain liquid over a broad operating temperature range.